Hammer layouts for impact pulverizers



March 18, 1958 w. M. SHELDON 2,327,242

HAMMER LAYOUTS FOR IMPACT PULVERIZERS Filed Sept. 9, 1955 0 0 0 0 of Z a one IN VEN TOR.

6/35 ATTORNEY United States 1 atent fiice 2,327,242 Patented Mar. 18, 1958 HAMMER LAYOUTS FOR IMPACT EULVERIZERS William M. Sheldon, Elizabeth, N. J., assignor, by mesne assignments, to Metals Disintegrating Company, inc, Elizabeth, N. J., a corporation of New Jersey Application September 9, 1953, Serial No. 379,149

1 Claim. (Cl. 241-486) This invention relates to hammer mills of the type in which a series of equally spaced hammers rotate in a grinding chamber having a circular inner wall in close proximity to the outer part of the hammer path and pulverizing methods using such mills.

This application is a continuation-in-part of application Serial No. 237,129, filed July 17, 1951, and thereafter abandoned.

Various forms of such mills are known. in some the lower part of the grinding chamber consists of a perforated or woven screen to retain the large particles and permit the acceptably fine material to pass out into a bag or other collecting device. Others have an annular grinding chamber from which air carrying the pulerized material is discharged radially inwardly through a centrifugal separator to reject the oversize. Grinding apparatus of this general type requires considerable power for its operation. This not only makes the grinding expensive, but also heats up the material treated.

With many substances, pulverization should be carried on without any great rise in temperature. One obvious effect of allowing the temperature to rise unduly is discoloration of the product due to chemical changes. For example, sugar, in the presence of even traces'of water, is decomposed at temperatures above 220 F., producing a dark-colored mixture of various bodies collectively termed caramel.

Another obvious effect of temperature rise occurs with materials such as cocoa. These materials become soft and sticky when warm. This change shows itself by the, building up of layers of cocoa over the interior of the mill, its screen and other parts.

The power used to drive a hammer mill is converted into heat which is dissipated by:

(1) Raising the temperature of the mill product.

(2) Raising the temperature of any air which passes through the mill.

(3) Raising the temperature of the mill and thereby that of the. ambient air.

Unless the flow of air is high, most of the heat goes into heating the product. Ten horsepower per hour is the equivalent of 25,450 B. t. u. per hour. This amount of heat, if transferred 100% to the product (taken as sugar) or 100% to the air passing through the mill, would raise the temperature of the product or air, re spectively, as given below.

Lbs. sugar per hour: Increase in temperature, F.

Cu. it. of air per hour:

To pass large amounts of air through a pulverizer, it is necessary to have an efficient centrifugal separator to prevent the coarse particles being swept out of the mill. With the ordinary screen type of separator, the air flow must be kept so low that efiicient elimination of heat by air is not possible.

In a hammer mill the material is broken by rotating hammers striking the particles repeatedly until the material has been reduced to the desired fineness. Now the force of the blows struck by the hammers on such material is proportional to the ditference in velocity be tween the hammers and the particles struck thereby. As the material to be pulverized is largely air-borne, the greater the movement of the air in the direction of movement of the hammers, the less effective become the blows of the hammers. The velocity with which the air rotates for any given hammerspeed and area of hammer face seems to depend upon the circumferential spacing of the hammers carried by the rotor. The more widely the hammers are spaced, the lower the velocity of rotation of the air.

Much of the power input is spent in agitating the air in the mill chamber. The less the air is. agitated, the less heat is generated. Increasing the spacing of the hammers by reducing their number reduces the rotation of the air in the mill and thereby increases the average efiectiveness of each impact and, at the same time, reduces the power spent on agitating the air inside the mill so that the additive effect of these various changes keeps the temperature down.

Tests made have revealed that, with sugar, reducing the number of hammers not only cut down the temperature in the mill but also increased the output of the mill.

A series of tests on sugar were made in an 11.5" internal diameter mill using respectively 12, 6, 3 and 2 equally spaced hammers. In each test the rate of feed of sugar was adjusted so that the power required to drive the pulverizer was approximately 10 horsepower. The output of the pulverizer increased remarkably as the number of hammers was reduced. This is clear from the following table.

Product through 200 mesh, percent Number of hammers Lbs. sugar per hour Similar tests using silica. sand failed to show marked increase in output when the number of hammers is reduced.

Since, whether the material to be ground was sugar or sand, reducing the number of hammers would have the same effect on air circulation and on the velocity of the hammers relatively to the particles to be broken, further tests were made to see whether the increase in output in the above sugar tests was due primarily to the drop in mill temperature with decrease in the number of hammers.

Two tests were made on an 8" internal diameter mill. In one test sugar alone was used, while in the other test sugar mixed with Dry Ice in the proportion of lb. Dry Ice to each 5 lbs. of sugar was employed. The rotor in both tests had six hammers. The air flow was the same in both cases. In each test the rate of feed of sugar was adjusted so that the power required to drive the pulverizer was approximately 5 horsepower.

The results were:

The above tests show that cooling the mill by adding small amounts of Dry Ice increased the output of sugar nearly 100%. As all conditions other than temperature remained constant, it appears that the increase in output with Dry Ice was due to sugar being much more brittle (i. e., its particles shatter more readily when struck) at relatively low temperature than at higher temperatures.

Apparently the physical characteristics of silica sand, such as its brittleness, change only slightly over the ternperature range 60 to 200 F., while sugar loses its brittleness markedly as the temperature rises through that range and also becomes more brittle when cooled below room temperature than at such temperature.

The proportion of air pushed forward in front of the hammers to that which flows radially inwards clear of the hammer path is a function of the depth of the hammer face measured radially with respect to the axis of the rotor. Consequently, the smaller the radial depth of the hammer face, the greater the velocity of the hammers with respect to the velocity of the particles carried by the air in the same direction as the hammers. For this reason the hammers should be of the stirrup type, with their long axes parallel to the grinding chamber wall, instead of the knife type with their long axes radial with respect to the axis of rotation of the rotor on which they are mounted.

The limitation on the extent to which the radial depth of the hammer faces may be reduced is that the depth must not be so small that much of the particles to be broken are not in the path swept by the hammers. The material flowing radially inward from one hammer must return, as the result of the action of the air stream and centrifugal force, to the path swept by the following hammer if the latter is to act thereon. Most of the particles appear to be concentrated in a shallow layer around the inside of the hammer chamber. This is due to the fact that rotational velocities much below the speed at which the rotor revolves create centrifugal forces greater than the force of gravity, so that there is a strong tendency for the material to be pulverized to accumulate at and to travel around the inner periphery of the grinding chamber. The fact that the hammers wear far faster at the outer corner nearest the chamber Wall than at other locations indicates that a very large part of the material is in a zone extending inwardly only a slight distance, say not more than 5% or at the most of the diameter of the chamber. The path swept by the hammers should coincide as far as practicable with the zone containing the larger part of the material to be pulverized. Accordingly, the radial depth of the hammer faces should preferably be not more than 5% or at most 10% of the diameter of the chamber.

The increase in efliciency with decrease in depth of hammer face is shown by the following table.

Lbs. of sugar per hour Power Width of hammer face lliipli t,

These figures were obtained with a mill having an 11.5" diameter chamber. Two rows of hammers were used, each row being spaced 180 apart while the hammers in one row were spaced 90 with respect to the hammers in the other row.

Staggering the hammers in the two rows increases the grinding eificiency somewhat, but the effect is much less than that obtained by increasing the spacing between the hammers in each row.

One suitable form of construction is shown, by way of example, in the accompanying drawings, wherein:

Fig. l is a vertical transverse section on the line 1-1 of Pig. 2 of a hammer-type pulverizer embodying the present improvements; and

Fig. 2 is a vertical longitudinal section of the same on the line 22 of Fig. 1.

The pulverizer comprises a casing 10 consisting of a scmicylindrical top portion 11 and a rectangular bottom portion 12 carrying a semicylindrical screen 13 to permit the pulverized material to pass out of the mill chamber. Within the casing mounted coaxially therein is a shaft 15 carrying a rotor 16 to which a series of stirrup hammers 17 are pivotally secured by pins 18. Bearings 19 for the shaft 15 are provided on either side of the casing. Material to be pulverized is placed in the hopper 21 and fed into the grinding chamber by three feed screws 22.

As shown, the rotor carries two rows of hammers each with two diametrically spaced hammers. For mediumsized mills, two hammers per row give better results than three. The hammers in one row are staggered with respect to those in the other row, the spacing being 90 as between the hammers in the two rows. As pre viously explained, excellent results are obtainable without staggering the hammers in the two rows.

In the mill illustrated the radial depth of the hammer faces is about 4% of the internal diameter of the grinding chamber. The arm portions of the hammers and the flanges on the rotor are narrow in longitudinal cross-section (see Fig. 2) so as to permit the rotor and hammers to rotate with minimum disturbance of the air in the mill chamber. With the construction shown, the air pushed out of the way by the hammers has plenty of space to flow inwardly around the hammers instead of rotating around the mill chamber along with the hammer which the air would tend to do if the hammers possessed a greater proportional radial depth.

The term stirrup-type hammer is used in the claim to include all hammers of the type having their long axes parallel to the inner surface of the grinding chamber and having arm portions which are narrow in longitudinal cross-section so as to permit the hammers to rotate with minimum disturbance of the air in the mill chamber.

I claim:

A high-speed hammer mill for pulverizing materials Whose brittleness decreases with rise in temperature and vice versa, comprising a cylindrical grinding chamber having a semi-cylindrical imperforate upper half and a lower half comprising a semi-cylindrical perforate screen to retain the large particles and permit the acceptably fine material to pass out of said chamber, a rotor, stirrup-type hammers having their long axes parallel to the inner surface of the grinding chamber and having arm portions which are narrow in longitudinal cross-section, such hammers being positioned parallel to the rotor axis and mounted thereon to sweep a cylindrical path only immediately adjacent the inner wall of the grinding chamber, the upper portion of the periphery of the grinding chamber having a substantially radial opening for the admission of material to be pulverized directly into said cylindrical path, sealing means for positively me chanically advancing such material through said open ing, the rotor carrying a single pair only of such hammers spaced apart in the same plane of rotation, the depth of the hammer faces in a radial direction with respect to the rotor axis not being in excess of 10% of the radius of the grinding chamber while the running clearance between the hammers and the chamber wall is less than the radial depth of the hammer faces, so that the air in the inner part of the chamber is disturbed by the hammers to a minimum extent, while the path swept by the hammers substantially coincides with the zone containing the larger part of the material to be pulverized.

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